1. Field of the Invention
This invention relates, in general, to a process for making gradient materials, more particularly, to a process for making gradient propellant materials, and most particularly, to a process for making gradient propellant materials using a twin-screw extrusion process.
2. Description of the Related Art
There is a great deal of interest in tailoring structures so the functional requirements can vary with location. In most cases, this will involve varying the materials that are used at specific locations within the structure resulting in discrete interfaces throughout. These discrete interfaces are often weaker than the surrounding materials and also act as stress concentrators, a dangerous combination that can lead to structural failure. Attempts at reducing stress concentrations and increasing the fracture toughness of interfaces have led to the concept of Functionally Graded Materials (FGMs). FGMs are structures that possess gradual variations in material behavior that enhance material and/or structural performance.
For example, at one point the material may be hard and at another point it may be soft. The description of this functional variation is known as the gradient architecture. The current challenge for manufacturing graded materials is to develop scalable processes that can easily control the continuous evolution of the gradient architecture within a structure in order to optimize structural performance. A power-law description of the gradient architecture for two-phase composite materials is conventionally used as follows the formula:V=(x/t)p where V is the volume fraction of one phase, x is the distance along the graded region (known as the interlayer), t is the thickness of the interlayer, and p is the gradient exponent. Values of p can range from 0 (all second phase in interlayer) to infinity (all first phase in interlayer).
A number of manufacturing technologies have been proposed for the processing of graded materials. They can be categorized as either transport-based or constructive processes. Constructive manufacturing processes that have been currently used to manufacture FGMs include: powder densification, coating, and lamination. Transport-based processes include: mass transport, thermal diffusion, centrifugal separation, and melt infiltration. While these techniques have been applied to metal and ceramic composites, there has not been as much research conducted on the manufacturing of graded materials using polymer composites.
One example of a current manufacturing technique for graded materials using polymer composites results in grading the distribution of SMA wires in polyurethanes using a lamination technique in order to control bending actuation for smart structure applications. A second example is using a technique for creating a continuously graded particle-reinforced polymer consisting of soda-lime glass microspheres in epoxy using a gravity casting technique to serve as a model material system for studying the failure of graded interfaces. However, both of these examples represent efforts to fabricate gradient architectures in polymer composites using laboratory-scale manufacturing techniques that are limited in the range of gradient architectures that can be produced for investigating the physics of graded materials. Thus, there is a need to demonstrate the production of continuous gradient architectures in polymer composites using industrial-scale manufacturing technologies that are capable of handling a variety of material systems and can produce a wider range of gradient architectures.